Illumination device and method for laser projector

ABSTRACT

Systems and methods for providing illumination suitable for imaging devices such as laser projection systems. In one embodiment, a highly collimated (e.g., laser light) beam is passed through a holographic diffuser to create a well defined cone angle for the light emanating from each point on the diffuser. This light is focused into an illumination image that is controlled by the prescription of the diffuser. In one embodiment, the image is a uniformly intense rectangle having a 4:3 aspect ratio to match an imager for a projection display. The diffuser prescription and resulting illumination image can be selected to match any desired imager. The present systems and methods may provide the advantages of high level of light efficiency, reduction or elimination of speckle and worminess and reduction or elimination of cosine 4  and gaussian intensity falloff, all of which are common in prior art designs.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the following U.S. provisionalapplications under 35 U.S.C. 119(e) which are all incorporated byreference as if fully set forth herein: Ser. No. 60/257,061, filed onDec. 20, 2000 entitled “Method and Appartus for Combining ParallelCollimated Lightbeams”, Attorney Docket No. RIAKE1100;Ser. No.60/257,047, filed on Dec. 20, 2000 entitled “Method and Appartus forCombining Parrallel Collimated Lightbeams”, Attorney Docket No.RIAKE1120;Ser. No. 60/257,062 filed on Dec. 20, 2000 entitled “Methodand Appartus for Elimating Zero-Order Light Leak in an IlluminationDevice”, Attorney Docket No. RIAKE1130;Ser. No. 60/257,063, filed onDec. 20, 2000 entitled “Method and Apparatus for Providing anIllumination Source Using a Segmented Diffuser”; Attorney Docket No.RIAKE1140;Ser. No. 60/257,045, filed on Dec. 20, 2000 entitled “Methodand Apparatus for Combining Polychromatic Light Beams Using anAchromatic Diffuser, Attorney Docket No. RIAKE1150;Ser. No. 60/257,046,filed on Dec. 20, 2000 entitled “Illumination Device Using MultipleLaser Light Sources and Having Zero-Order Light Leak Correction,Attorney Docket No. RIAKE1160;Ser. No. 60/284,555, filed on Apr. 18,2001 entitled “Method and Apparatus for Providing SelectableIllumination Sources”, Attorney Docket No, RIAKE1170;Ser. No.60/282,738, filed on Apr. 10, 2001 entitled “Polychromatic DisplayDevice Using Monochromatic Diffusers, a Beamsplitter and a Combiner inan Optical Processor Space”, Attorney Docket No. RIAKE 1200;Ser. No.60/282,736, filed on Apr. 10, 2001 entitled “Method and Apparatus forCombining Multiple Monochromatic Images Using an Optical ProcessorSpace”, Attorney Docket RIAKE1210;Ser. No. 60/282,735, filed on Apr. 10,2001 entitled “Monochromic Display Device Using a Monochromatic Diffuserand a Beamsplitter and a Combiner in an Optical Processor Space”,Attorney Docket No. RIAKE1250;Ser. No. 60/282,737, filed on Apr. 10,2001 entitled “Polychromatic Display Device Using a Chromatic Combiner,and Achromatic Diffuser and a Beamsplitter and a Combiner in an OpticalProcessor Space”, Attorney Docket No RIAKE1260;Ser. No. 60/282,734 filedApr. 10, 2001 entitled “Polychromatic Display Device Using MonochromaticDiffusers, a Beamsplitter and a Combiner in an Optical Processor Space”,Attorney Docket No. RIAKE1270 and claims benefit of Ser. No. 60/222,301filed Aug. 1, 2000.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates to projection displays and more particularly animproved method of homogenizing and formatting the light from a lightsource to produce higher uniformity and efficiency in the projectedrange.

2. Description of Related Art

Illumination systems used for image projectors are designed to generatea spatially uniform plane which can be used to illuminate an imagingdevice film or other media. The reflected or transmitted light from theimaging device is then projected onto a screen for viewing. Thebrightness and spatial brightness uniformity should be within certainlimits for each particular application to be considered acceptable tothe viewers.

Image projectors including film movie projectors, slide projectors,electronic liquid crystal and micro-electro-mechanical (mem) projectors,microfilm and overhead projectors all require a high degree of spatiallight uniformity in the image to produce a pleasing image. This hasalways been a challenge for projection system designs due to the factthat the light sources available for these systems all have verydisorganized light output and therefore require complex optical systemsto organize the light. Additionally, high degrees of magnification inshort distances (which often occur in these optical systems) cause aproblem which is well known in the optical field—the cosine⁴ roll off ofpower in the image as you move radially away from the center of theimage. This effect is most predominant at the corners of the image.Another problem is that light sources tend to produce round orelliptical gaussian beam profiles, while most images are rectangular informat. Typically, the light beam is spatially truncated (i.e., theportions of the beam which fall outside a rectangular profile thatcorresponds to the image are blocked). This leads to another problem,which is maximizing the brightness of the illumination—when the light istruncated to change its geometry, the truncated light is obviouslywasted.

Many optical methods have been used in the prior art to try to minimizethe variations in uniformity which are due to the particularcharacteristics of the available light sources as well as to maximizethe brightness of the illumination.

The optical method used depends somewhat on the light source used. Manydifferent types of light sources are in common use today. Some types areelectric filament, and arc lamps including metal halide arc, low andhigh pressure mercury arc, xenon arc, carbon arc, as well as solid stateLight Emitting Diode (LED) sources, and Lasers. Not all of these lightsources, however, are suitable for displays using prior arttechnologies.

Two of the most common types of light sources in use in commercialapplications are metal halide arc lamps and high pressure mercury arclamps.

These arc lamps are usually configured in an optical illumination systemwhich employs an elliptical or parabolic reflector to gather and directthe light to a focal point or collimated beam respectively, as shown inFIG. 1. Both of these types of systems produce highly non-uniform beams.Some systems use reflective tunnels or light pipes through which thesource light is channeled in order to create a scrambled, hence morespatially uniform bundle of light rays as shown in FIG. 2.

Lenslet arrays are also sometimes used to increase the uniformity of thelight. Some versions of these lenslets are described in U.S. Pat. Nos.5,098,184 and 5,418,583. The lenslet arrays function essentially in thefollowing manner. Two lenslet arrays are separated by a distance equalto the focal length of the individual elements. The elements of thefirst array form an image of the source in the aperture of the elementsof the second array. In the case of a laser, the source image is adiffraction pattern. The elements of the second array then form an imageof the aperture of the elements of the first array on the illuminationplane. The aperture is chosen to match the aspect ratio of the device(film gate, or LCD) to be illuminated. A field lens in close proximityto the second array focuses the chief rays of each element to the centerof the illumination plane so that the subsets of the beam sampled by allelements of the arrays are superimposed at the illumination plane and anaveraging process thus occurs that causes the illumination plane to havemore uniform irradiance. A second field lens is often required at theillumination plane to ensure that the light is telecentric as most oftenrequired by projection imaging optics.

In this manner a beam with non-uniform irradiance may be sampled byarrays composed of many elements and converted to a uniform beam with adifferent geometry (generally rectangular).

The lenslet array optical system which is used in an illumination systemhas design characteristics that must be adjusted to ensure that theillumination and imaging systems are compatible. If they are not, thenlight is wasted. For example, the geometry of the illumination should bethe same as the geometry of the imager. The numerical aperture of theillumination system should also be compatible with the imaging system.The ratio of the footprint of light incident on the first array to thedistance to the illumination plane determines the numerical aperture ofthe illumination light. Thus the focal length of the array elements andthe field lens focal lengths are adjusted to ensure that theillumination numerical aperture matches the imaging numerical aperture.

At first blush, laser light appears to have enormous potential for beingthe illumination source in projection display systems. The light is wellbehaved and organized (ie: it is collimated), it is chromatically pure,and with a minimum of three wavelengths (Red, Green, and Blue) a highcolor space or gamut can be created, and high power low cost lasers arebecoming available. There are, however, several problems withlaser-based illumination systems.

First, the coherency of laser light leads to speckle, which is afine-grained non-uniformity. The speckling effect is increased with theuse of so-called holographic diffusers as proposed in this invention.The net effect is often a high frequency mottling effect sometimescalled worminess. Another problem is that the laser light is collimatedand, as such, it is difficult to create a cone or numerical aperturewhich will allow an image to be projected onto a screen, as with aprojector. Yet another problem is that the laser light typically has agaussian intensity profile and it may have a wide range of diameters,depending upon the particular laser source which is used. This can, andoften does, lead to a non-uniform light distribution on the final screenor projected image surface.

Another problem is that currently available lasers typically do not haveenough power to provide sufficient illumination in some display devices.Further, using prior art methods, it is difficult to combine the beamsof multiple lasers to obtain sufficient illumination for this purpose.

Another problem with the use of laser light as a display illuminationdevice is that the beam generated by a laser may be astigmatic in itsdivergence. In other words, the divergence in the beam”s cross sectionmay be greater in one axis than another. This causes additionalprocessing problems compared to a circularly symmetric diffractionlimited beam.

Yet another problem with the use of laser light in a displayillumination device is that, if laser light is diffracted in an opticalsystem, a certain amount of light passes through the diffracting devicewithout being diffracted. This effect is referred to as zero-order lightleak. Zero-order light leak may prevent the resulting diffractionpattern from conforming to a well-defined, desired function. Anotherproblem with using laser light sources for illumination is that they aremonochromatic. Since it is desirable to have a source of white light, itmay be necessary to combine laser light beams of several differentwavelengths (e.g., red, green and blue.) This may be difficult becausemany optical systems and components are wavelength-dependent and maytherefore require color correction to provide even illumination.

Another problem with the use of laser light in display systems is that alarge physical volume is normally required. The space requirements ofthese systems results in part from the separate processing of the laserillumination light in a first optical system and the subsequentprocessing of the image information in a second optical system so thatit can be displayed for viewing.

Yet another problem with the use of laser light in a displayillumination device is that optical processors for formatting theillumination image from the laser source are configured to provide asingle fixed illumination aspect ratio format. To obtain a differentaspect ratio format for use in the display, the illumination source istypically masked, so a portion of the light is lost and significantsystem efficiency is lost. In order to utilize all of the lightgenerated by the laser source, it may therefore be necessary to use anentirely different optical processor.

SUMMARY OF INVENTION

One or more of the problems outlined above may be solved by the variousembodiments of the invention. The present invention performs a similarfunction as a lenslet array optical system, but does so moreeffectively, with fewer and lower cost components, and with improveddesign flexibility. The present techniques may be applied to many typesof illumination sources such as arc lamps and LED”s in addition tolasers.

Broadly speaking, the invention comprises a system and method forconverting a laser beam having a non-uniform profile into a source ofillumination which has uniform power density. The generated illuminationimage may be used for a variety of purposes. For example, the image maybe a uniformly intense rectangle suitable for use in a display device,or it may be a round dot suitable for transmitting the light into anoptical fiber. The present invention can be used to conserve the powergenerated by the laser source and direct substantially all of the powerinto the desired illumination region. Laser speckle artifacts can alsobe reduced or eliminated at the same time. The choice of design of theelements in the system allows for precise control of the illuminationpattern and the particular telecentric cone angle patterns exiting theillumination pattern. While the preferred embodiment uses a lasersource, the system is capable of utilizing a wide variety of lightsource devices, including all arc lamps and LED sources.

The operation of a system in accordance with one embodiment of theinvention is as follows. A block diagram of the system is shown in FIG.4. A beam of light is first generated by the laser light source. Thelight beam is expanded or sized to illuminate a controlled anglediffuser. The expanded beam remains collimated.

The expanded beam is passed through a controlled angle diffuser (e.g.,hologram, bulk scatterer, etc.) to diffract or direct the light in apredetermined pattern. (Crossed lenticular arrays, or lenslet arrays canalso be used.) The controlled angle diffuser can be designed to emitlight angularly in any geometry (such as rectangular to match a displaydevice aspect ratio). The angular emission of a holographic diffuser issimilar to the aperture geometry of the lens array system describedabove. It should be noted, however, that in the prior art it takes twooptical elements with an intervening space to produce an effect which isperformed by a single optical element (the holographic diffuser) in thepresent system.

A first field lens is positioned following the holographic diffuser.This first field lens focuses and spatially overlays the diffractedlight onto a single rectangular plane which lies at a distance from thelens equivalent to its focal length. A second field lens is used at thisillumination plane to correct for the degree of telecentricity desiredin the system. In some cases, over-correction or under-correction may bedesired. This image is then used as the illumination source for adisplay. Both field lenses function identically to field lenses in lensarray systems, but at significantly lower cost.

The present systems and methods may provide a number of advantages overprior art. For instance, the level of light efficiency may besubstantially increased over the prior art. Further, the problems oftenencountered in coherent optical systems relating to speckle and imageworminess (high frequency intensity variation) may be reduced oreliminated. Another advantage is that the illumination provided in thismanner is uniform and can be spatially formatted to match the displaydevice being illuminated (rather than providing illumination with thegaussian intensity falloff which is common in prior art designs).

An alternative to the holographic diffuser is a crossed lenticular arrayas shown in FIG. 5A. The crossed lenticular array performs the sameoptical function as the hologram for a rectangular emission profile, butat a lower spatial sampling rate. The lens profiles in the lenticularcan be aspheric to compensate for uniformity issues as described above.The crossed lenticulars can be combined into one element as shown inFIG. 5B. An additional configuration is to integrate the crossedlenticular function into a single element lenslet array as shown in FIG.5C. While the lenslet arrays reduce the beam sampling rate and therebyslightly reduce the resulting image uniformity, they are significantlymore achromatic than holographic diffusers and can therefore be usedwith polychromatic light sources. This embodiment also provides asignificant advantage over the prior art in that it does not require theintervening space and volume between the prior art lenslet arrays andthereby allows for construction of more compact systems.

BRIEF DESCRIPTION OF DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredin conjunction with the following drawings.

FIG. 1 is a diagram illustrating ellipitical and parabolic arc lamps inthe prior art.

FIG. 2 is a diagram illustrating an elliptical arc lamp and light tunnelhomogenizer in the prior art.

FIG. 3 is a diagram illustrating a lenslet array in the prior art.

FIG. 4 is a functional block diagram of an illumination system inaccordance with one embodiment of the invention.

FIG. 5A is a diagram illustrating a lenticular array.

FIG. 5B is a diagram illustrating a compound crossed lenticular.

FIG. 5C is a diagram illustrating an integrated crossed lenticular

FIG. 6 is a diagram illustrating a preferred embodiment of theinvention.

FIG. 7 is a set of diagrams illustrating an optical system designed toexpand a light beam from a diode edge emitter laser beam optics byvarying degrees in orthogonal planes.

FIG. 8A is a diagram illustrating the profile of a cone of lightemerging from a holographic diffuser in one embodiment of the invention.

FIG. 8B is a diagram illustrating the profile of a cone of lightemerging from a holographic diffuser in an alternative embodiment.

FIG. 8C is a diagram illustrating the profiles of several cones of lightemerging from a holographic diffuser in one embodiment.

FIG. 9 is a diagram illustrating the specific design of an illuminationsystem in a preferred embodiment.

FIG. 10 is a flow diagram illustrating the operation of an illuminationsystem in accordance with one embodiment of the invention.

FIG. 11A is a diagram illustrating a prior art transmissive imagersystem.

FIG. 11B is a diagram illustrating an embodiment of the present systemincluding a transmissive imager.

FIG. 12A is a diagram illustrating a Polarizing Beam Splitter/Imagersystem in the prior art.

FIG. 12B is a diagram illustrating a Polarizing Beam Splitter/Imagersystem in accordance with one embodiment of the invention.

FIG. 13A is a diagram illustrating a Prior art one color sequentialimaging system.

FIG. 13B is a diagram illustrating a color sequential imaging system inaccordance with one embodiment of the invention.

DETAILED DESCRIPTION

A preferred embodiment of the invention is described below. It should benoted that this and any other embodiments described below are exemplaryand are intended to be illustrative of the invention rather thanlimiting.

In broad terms, the present invention comprises a system and method forprocessing a laser light beam in an optical system that uses acontrolled angle diffuser to produce an image of predetermined shape andintensity.

Referring to FIG. 6, a preferred embodiment of the invention is shown.The invention comprises a laser light source 1, a beam expansion andcollimating section 2, a holographic diffuser 3, a first field lens 4,and a second field lens 5. In this embodiment, all elements arecoaxially centered. The function of the optical processing by thecomponent elements is to convert the incoming substantially collimatedround Gaussian laser beam to a uniform rectangular illumination plane 6for use in illuminating a spatial light modulator such as a liquidcrystal display panel (or any other type of imager). The spatial lightmodulator can either be illuminated immediately following the field lens5 or the illumination plane 6 can be optically relayed with or withoutmagnification to another position in the system.

The laser light source in one embodiment may comprise an edge emittinglaser. Typically, such a laser emits light in a pattern which hasdifferent orthogonal divergences. That is, the emitted beam divergesmore in a first plane than in a second plane. The beam must therefore becorrected by an optical system (e.g., beam expander) which has adifferent prescription in the first plane than in the second. This maybe achieved in one embodiment using a pair of crossed cylindrical lensesof different powers as the diverging lens of the beam expander. Theconfiguration of the pair of cylindrical lenses in this embodiment isshown in FIG. 7. Referring to FIG. 7, it can be seen that the firstcylindrical diverging lens 2 c causes the beam to diverge in a firstplane, but not a second. The second cylindrical diverging lens 2 d, onthe other hand, causes the beam to diverge in the second plane, but notthe first. After the beam has passed through both of the cylindricaldiverging lenses, the divergence is equal in both planes and can becollimated by a converging lens. The beam exiting the beam expander istherefore collimated in both planes.

It should be noted that the cylindrical lenses described above may bereplaced in another embodiment by a single astigmatic lens whichperforms the same function (refracting the beam by different amountsalong different axes.) Likewise, the correction of the differentdivergences need not be corrected by the diverging lens(es). It mightinstead be corrected by a pair of cylindrical converging lenses, or byother elements in the optical system. In another embodiment, thedivergence of the beam from the laser light source might already havegreater divergence than desired in one plane so that one of thecylindrical lenses might be a converging lens while the other is adiverging lens. Many such variations are possible.

Light Emitting Diodes may also be used as light sources in otherembodiments. If an LED is used, an optical system which converts the LEDoutput profile to a substantially collimated beam is positionedfollowing the LED. Optical systems to accomplish this are well known inthe art.

The preferred embodiment would use a high power VECSEL (Vertical CavitySurface Emitting Laser) such as those manufactured by Novalux, Inc andtermed NECSEL (Novalux Extended Cavity Surface Emitting Laser) due toits substantially cylindrical beam shape and high power capability.

The ability to modify the system to operate with a wide range of sourcesand source intensity profiles is one of the advantages that may beprovided by the present system.

Laser light 1 is shown entering the system of FIG. 6 from the left. Thelight is monochromatic and collimated with a typical cylindrical beamdiameter of 0.3 3 mm, although other diameters and geometries arefeasible. Polychromatic sources such as tunable lasers or pre-combinedmonochromatic sources may also be used. While the intensity profile ofthe beam in the preferred embodiment is Gaussian, other intensityprofiles and laser multi mode profiles will work as well.

Once a substantially collimated light beam is established a beamexpander can be used to expand the beam diameter. The amount by whichthe beam is expanded is determined by the desired F number (as will bedescribed below). The beam expander may be omitted if the collimatedsource is of sufficient diameter.

A beam expander (2) expands the light beam and re-collimates the light.In a first embodiment, the beam expander is comprised of two elementsand an intervening beam expansion space. In this embodiment a firstplano-cave lens 2 a is used to create a conical beam divergencesymmetrically centered along the optical axis. A second piano-convexlens 2 b is used to halt the beam expansion and re-collimate the laserbeam into a second larger diameter beam having its divergence minimizedso that its rays are substantially parallel to the optical axis. Thislarger diameter beam is then directed onto a holographic diffuser (3).

A holographic diffuser (3) follows the beam expander. In the preferredembodiment this diffuser has the properties of converting an incidentlaser beam to a plurality of rectangular light cone profiles as shown inFIG. 8C according to the hologram prescription. That is, the lightexiting each differential point on the diffuser forms a rectangular coneof light. The rectangular cone of light has its horizontal and verticalorthogonal angles in the ratio of the format of the desired illuminationpattern for a display device. In the preferred embodiment, the desiredillumination pattern at the output is a uniformly intense rectangle of4:3 aspect ratio to correspond to standard NTSC television format andstandard XGA computer monitor format. In the specific design exampleshown in FIG. 9, the corresponding angles are theta_(Horiz)=20 degreesand theta_(Vert)=14.8 degrees. The specific horizontal and verticalangles for the 4:3 aspect ratio system or any other format arecalculated as follows:

theta_(Horiz)=Arctan (0.5×W _(Image) /D _(diff-image))

theta_(Vert)=Arctan (0.5×H _(Image) /D _(diff-image))

Where

theta_(Horiz)=diffuser horizontal half angle divergence

theta_(Vert)=diffuser vertical half angle divergence

W_(Image)=Half width of the desired Image plane 6

H_(Image)=half height of the desired Image plane 6

D_(diff-image)=Distance from diffuser to Image plane 6

Other hologram prescriptions would be used for wide format HDTV, etc.)Each of these light cones is generated from energy from a small section,or sample, of the laser beam Gaussian power profile resulting in a muchhigher level of uniformity in each light cone than in the original beam.In the preferred embodiment, the center ray of these cone patterns issubstantially parallel to the optical axis. Each ray within a givenexpanding cone has a corresponding parallel ray in all of the othercones being emitted from the surface. All of these parallel rays are atthe same angle relative to the central axis. Each set of parallel rayswill map to a unique point on the Illumination Plane 6, as a result ofthe field lens 4 described below. Therefore, the angular pattern of raydivergence defines the shape of the Illumination image at plane 6. Sinceeach point in the Illumination image will be composed of energy from allpoints in the incoming Gaussian beam, the uniformity of the illuminationPlane is substantially improved over the uniformity of the originalgaussian beam. The effect is similar to the prior art lenslet arraysystems whereby each rectangular cone of light is created by samplingthe incoming beam at all points and then overlaying the samples on eachother at the illumination plane. The Lenslet arrays sample a much lowerspatial frequency and therefore produce a less uniform result.

Other light cone profiles (e.g., circular) are also feasible as shown inFIG. 8B. In fact, the profile may be arbitrarily defined for theapplication.

The final uniformity is then dependent primarily on the angular powerprofile of the diffraction pattern of the holographic diffuser. In thepreferred embodiment, this profile is that of substantially linear powerper degree of solid angle to effect a near uniform power and intensityin the Illumination image. Nonlinear hologram power profiles versusangle of divergence of the light cones can be designed into the hologramto compensate for geometric uniformity problems in the illuminationpattern such as the cosine⁴ power rolloff or other systemnon-uniformities.

Referring to FIG. 8A, a diagram illustrating the diffraction of light ata single point on a holographic diffuser is shown. As the collimatedlight passes through the holographic diffuser, it is diffracted so thatit exits in a certain cone of light. (Cone refers to the solid angleinto which the light is radiated.) The cone may be irregularly shaped,as indicated by the dashed line at the right side of the figure if otherillumination plane formats are desired. This dashed line is the outlineof the diffraction pattern image. The diffraction pattern image ischaracteristic of the holographic diffuser, and the light emanating fromeach point on the holographic diffuser radiates outward in a cone of thesame shape (i.e., the shape of the image.) The holographic diffuser canbe configured to create any desired diffraction pattern (andcorresponding image.) Referring to FIG. 8B, a holographic diffuserconfigured to generate a rectangular image from each incident point isillustrated. It is contemplated that a holographic diffuser which isconfigured to generate this type of image will be useful in applicationssuch as projection-type displays, where a rectangular light source isdesired. More particularly, the holographic diffusers which are used indisplay devices can be configured to produce an image which is uniformlyintense across its entire area, thereby resulting in a higher-qualityimage on the display.

It should be noted that the dashed image outlines illustrated in FIGS.8A and 8B are not themselves images. They are instead representative ofthe cross-section of the cone into which light radiates from aparticular point on the holographic diffuser. Thus, light radiating froma different point on the holographic diffuser will radiate into anidentical cone which is displaced laterally from the illustrated cone.While the cones originating at each point on the holographic diffuserare displaced from each other, the image which is produced by passingthis light through a field lens and thereby focusing it does not movewith the addition of light emanating from new points on the holographicdiffuser. Instead, this additional light increases the intensity of theimage which has already been formed. The additional light may, howeveralter the angular extent of the image formed by the lens.

FIG. 8C shows some of the plurality of rectangular patterns generatedacross the hologram from the area illuminated by the laser beam.

The profile of the illumination footprint on the diffuser controls theangular extent of the light cones exiting the Illumination Plane (6) andthus the numerical aperture or F number of the system. Parallel raysfrom the diffuser pattern all map to a unique point on the IlluminationPlane. The exit angle of that ray from the Illumination Plane 6 isdetermined by the radial offset of that ray from the image point. Thecollection of rays which pass through the image point thereby set thelight cone shape and divergence corresponding to that point.

Therefore, the diffuser (3) solid cone angle shape (i.e., thediffraction pattern) defines the spatial extent of the Illuminationimage and the Laser Illumination footprint on the diffuser (3) definesthe shape of the light cones and the F number at the Illumination Plane(6.) In alternative embodiments, crossed lenticular lenses (FIGS. 5A and5B) or lenslet arrays (FIG. 5C) can be used in place of the holographicdiffuser. Both of these alternatives have more achromatic performanceand can be used more easily with polychromatic light sources, but theysample the source beam at a lower spatial frequency than a holographicdiffuser and may therefore reduce the uniformity of the illuminationimage relative to the Holographic diffuser embodiment. Aspheric lensletsurfaces may be used to tailor the angular power profile and therebyfurther improve the Illumination image uniformity.

A first field lens (4) follows the diffuser surface. The field lens 4maps each parallel ray from the diffuser to a unique point on anIllumination Plane 6 effectively performing and angle to areatransformation on the light exiting the diffuser. This process in effectoverlays each of the diffuser rectangular cones on each other in theIllumination Plane 6 producing a highly uniform image. The IlluminationPlane 6 is located one focal length from field lens (4) and, in thepreferred embodiment, produces a rectangular Illumination image having a4:3 aspect ratio. This should not, however, be considered a limitation,as other values may be viable in a given embodiment, depending on itsgeometry.

The physical lens may be a common single element lens or it may be arelief fresnel lens or a holographic fresnel lens. An advantage of usingany of the fresnel lenses is that they are lower cost and in some casescan be laminated to the diffuser for further assembly simplicity andcost reduction.

A second field lens (5) which has the same focal length as the firstfield lens is placed at the image plane (6) of the first field lens. Thefunction of this lens is to correct the divergence of the telecentriccone angles exiting the Illumination Plane. Without this lens, thecentroid of each light cone bundle exiting the Illumination Plane isdirected along a radial from the center of the first field lens (4.) Inother words, the centroid of the light cone lies on the line extendingfrom the center of the first field lens to the point at the image planewhich defines the vertex of the cone. By adding a second field lens (5)at the Illumination Plane (6) with a focal length equal to the firstfield lens (4) and having sufficient diameter to circumscribe the entireIllumination Plane image, the light cones exiting the Illumination Planecan be made to have their respective centroid exit angles substantiallyparallel to the optical axis. This lens can be either overpowered orunderpowered, that is its focal length may be adjusted as the imagingoptics system requires. This geometry will provide telecentric light inplane (7) which can then be imaged onto a display device such as areflective or transmissive LCD or similar device.

A specific design example of the preferred embodiment is shown in FIG.9.

A functional diagram of the preferred embodiment is shown in FIG. 10.

The optical system described above may be used for a number of purposes.One of these purposes is the illumination of an imager in a projectiondisplay device. It is desirable in such devices to have a source ofillumination which is uniform and which has a shape corresponding to theshape of the imager used in the device. In this instance, a holographicdiffuser which forms such an image can be selected. The optical systemcan then be configured to focus this image either on a plane which iscoincident with the imager of the display device, or on a plane fromwhich it can be transmitted, via relay optics, to the imager.

Several projection system utilizing the invention are shown in FIGS. 11and 12. These architectures are well known in the art and should beexemplary of how the invention can be used in such systems.

FIG. 11A shows a typical prior art system using an arc lamp using threeseparate imagers for each primary red, green and blue color and threetransmissive imagers system for each corresponding primary. In this caseoptical filters are used to separate the white light from the sourceinto its constituent primary colors.

FIG. 11B shows a three imager transmissive system which uses threeseparate imagers for each primary red, green and blue color with threeseparate monochromatic illumination sources which each comprise theinvention. In each of the separate illumination sources, the hologramprescription is designed to operate at a specific monochromaticwavelength so as to produce the same size illumination image to fit thespatial light modulator(the imager) each of which are the same size andshape. In the case of three imager systems, all sources are oncontinuously.

FIG. 12A shows a typical prior art three polarizing beam splitter systemusing an arc lamp using three separate imagers for each primary red,green and blue color and three transmissive imagers system for eachcorresponding primary. In this case optical filters are used to separatethe white light from the source into its constituent primary colors.

FIG. 12B shows a three imager, three beamsplitter reflective imagersystem using three independent sources comprising the invention asdescribed above.

FIG. 13A shows a typical prior art one imager color sequential systemusing an arc lamp source and a color filter wheel for temporal colorsequencing. The sources are temporally modulated in sequence with thecolor information active on the spatial light modulator(the imager.)FIG. 13B shows a one imager color sequential system also using threeindependent sources comprising the invention pre-combined by a colorcombiner to produce a coaxial polychromatic illumination source. Thesources are temporally modulated in sequence with the color informationactive on the spatial light modulator(the imager.)Another purpose forwhich the present system can be used is the combination of laser lightbeams for input to an optical fiber. Laser light sources are currentlyused in fiber optic communication systems to provide optical signalswhich are input to the fibers. Often, however, these laser light sourcesdo not provide sufficient power to transmit signals over the desireddistances. Using the present system, a plurality of laser light beamscan be combined for input to a single fiber. In this instance, adiffuser which images the light beams as a single spot smaller than thediameter of the fiber can be selected. The spot can be imaged onto theend of the fiber, thereby transmitting the light into the fiber. In thisembodiment, the aperture of the diffuser and/or corresponding field lenscan be selected to ensure that the light which is imaged onto theoptical fiber is within the numerical aperture necessary to transmit thelight into the fiber.

The benefits and advantages which may be provided by the presentinvention have been described above with regard to specific embodiments.These benefits and advantages, and any elements or limitations that maycause them to occur or to become more pronounced are not to be construedas a critical, required, or essential features of any or all of theclaims. As used herein, the terms “comprises,” “comprising,” or anyother variations thereof, are intended to be interpreted asnon-exclusively including the elements or limitations which follow thoseterms. Accordingly, a process, method, article, or apparatus thatcomprises a list of elements does not include only those elements butmay include other elements not expressly listed or inherent to theclaimed process, method, article, or apparatus.

While the present invention has been described with reference toparticular embodiments, it should be understood that the embodiments areillustrative and that the scope of the invention is not limited to theseembodiments. Many variations, modifications, additions and improvementsto the embodiments described above are possible. It is contemplated thatthese variations, modifications, additions and improvements fall withinthe scope of the invention as detailed within the following claims.

What is claimed is:
 1. A system comprising: a light source configured toemit a highly collimated light beam; a controlled angle diffuserconfigured to receive the highly collimated light beam and to generate adiffraction pattern therefrom; and a field lens configured to focus thediffraction pattern into an image; and a field lens configured tocorrect the telecentricity of the image.